Electronic and Structural Properties of the Spin Crossover Systems Bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) Thiocyanate and Selenocyanate

Abstract

Essentially high-spin
[Fe(bpp)2][NCS]2·2H2O
and [Fe(bpp)2][NCSe]2 (bpp =
2,6-bis(pyrazol-3-yl)pyridine) were isolated from an aqueous reaction mixture.
Both salts undergo an abrupt transition to low spin below room temperature,
that for the thiocyanate occurring in two steps and the high-spin Æ
low-spin Æ high-spin cycle being accompanied by hysteresis in both
steps. Recrystallization of the salts from nitromethane yielded a mixture from
which bright yellow crystals were separated for structure determination. In
addition, from the recrystallized selenocyanate, deep red-brown crystals of
composition
[Fe(bpp)2][NCSe]2·H2O·0.25
CH3NO2 were obtained.
Recrystallized
[Fe(bpp)2][NCS]2·2H2O
and
[Fe(bpp)2][NCSe]2
were identified as high spin with average Fe–N distances of 2.16 and
2.17 Å, respectively. In the unit cell of
[Fe(bpp)2][NCSe]2·H2O·0.25
CH3NO2, there are four independent
iron atoms, three identified as low spin and the fourth as high spin. All
salts crystallize in a layer-type array involving edge-to-face and
face-to-face aryl–aryl-type interactions. Hydrogen bonding between
pyrazole >NH groups, anions and solvate molecules is observed. The
structure of the uncoordinated ligand was also determined, the molecule being
found in a planar arrangement with
thecis–cis configuration for
the pyrazolyl groups relative to the central pyridyl and the >NH group
being at the N 2 atom. Hydrogen bonding involving the >NH groups leads to
stepped stacks of molecules. The principal difference in the geometry of
coordinated and free bpp molecules is a contraction in the angles about the
interannular bridges in the chelate rings.
[Fe(bpp)2][NCS]2·2H2O:
triclinic, space group P1–,
a 8.302(6), b 8.446(6),
c 21.531(13) Å, a 78.78(5), b 82.80(5), g
89.85(4)˚, Z 2.
[Fe(bpp)2][NCSe]2:
triclinic, space group P1–,
a 8.354(4), b 8.409(4),
c 19.918(9) Å, a 87.02(3), b 83.15(3), g
88.86(3)˚, Z 2.
[Fe(bpp)2][NCSe]2·H2O·0.25
CH3NO2: monoclinic, space group
Pn, a 16.425(12),
b 20.774(9), c 16.933(14) Å,
b 90.91(4)˚, Z 8. Uncoordinated bpp: orthorhombic,
space group Pna21,
a 8.075(3), b 22.479(9),
c 5.525(1) Å, b Z 4.